This invention relates to a new and improved method and apparatus for rapidly sharpening knives and similar tools to create a superior cutting edge. As used herein, the term knife shall be defined to include any sort of blade such as chisels, plane edges, scissors, razor blades, and similar precision edges or cutting tools.
There are a wide variety of known means for sharpening knives some of which are discussed in the copending U.S. applications cited above. The large number and wide variety of existing means discussed in that application for sharpening knives is testimony to the complexity and difficulty of sharpening knives in a fast, convenient, and satisfactory-way that will consistently produce a sharp cutting edge. There is today in fact no known available means for the unskilled to produce rapidly and consistently razor-like cutting edges on knives.
Rapid sharpening requires a means to remove rapidly the material of composition of the knife--often a high carbon steel or a stainless steel. The rate of metal removal is related to the inherent hardness of the abrasive used, the particle size, or grit as it is commonly called, of the abrasive, the applied pressure on the knife edge, and the linear velocity of the abrasive particles across the edge being formed or sharpened. The hardest material commonly used for metal removal is diamond with a hardness of 10 on the Mohs' scale, compared to about 5.5 or so for many steel alloy knives. Other materials such as alumina, high density alpha alumina, carborundum, certain natural stones and the like also are harder than most steels and hence can be used for sharpening through abrasive action against the metal.
Creation of the finest cutting edges on the order of one tenthousandth (1/10,000) of an inch in width can be accomplished with these abrasive compositions, but a fine grit must be used and the velocity of the abrasive must be held below a critical limit to avoid overheating the thin and fine edge being created by the abrasive action. An abrasive system and apparatus designed to create fine edges such as that described in the copending applications cited above will remove metal at a rate lower than a system where the abrasive particles are larger and moving at higher velocities.
Because creation of the finest cutting edges involves inherently a slower metal removal rate, any process designed to create such edges is not optimum for the task of initial metal removal such as where a knife is first being formed or where the blade is particularly dull. Consequently, to reduce the total elapsed time needed with a very dull knife to create a thin and fine edge of a thickness limited only by the composition of knife and its crystalline structure, one usually resorts to a series of different and time consuming grinding and sharpening operations. None of the integrated sharpening equipment existent today are satisfactory for the rapid generation of fine edges on the order of 1/10,000 inch on otherwise very dull knives.
Much prior art has been concerned with disk type sharpeners for rapid sharpening such as described in U.S. Pat. No. 3,680,264. They have proved unsatisfactory because of serious control problems inherent with disks which manifest difficulties in positioning the knife accurately, in controlling the angular relationship of the knife with the disk face, and in creating excessive heating of the knife edge during sharpening. A most serious disadvantage has been the tendency of the disk to "grab" the knife when its edge is rested on the flat surface of the disk and to grind undesirable scallops or grooves along the knife edge in an uncontrolled manner. Such grabbing occurs if there is instability in the control of the angle that the knife face makes with the disk face, or inadequate means to hold the knife edge parallel to the flat surface of the disk, or poor control over the consistency of force applied to the knife edge by the disk or operator during sharpening.
A major cause of poor sharpening with disk sharpeners is poor control of knife angle relative to the rotating disk such as exemplified in prior art U.S. Pat. No. 2,496,139 that actually allows the knife guide to wobble and the sharpening angle to be determined more by operator skill or by the knife width and thickness. Poor control of the knife edge parallel to disk face or poor control of the angle of knife face relative to the principal plane of a disk sharpener is unacceptable if one wishes to optimize blade edge sharpness and to avoid gouging.
To minimize such uncontrolled gouging and grabbing of knives sharpened with disks, the prior art commonly has resorted to maintaining contact of the knife edge only with the corner edge of the disk such as described in U.S. Pat. No. 3,334,446 and deliberately avoiding a planar contact between the knife edge facet and the disk face perpendicular to its axis of rotation. In that patent the described disk is spring loaded to help reduce gouging and the knife is positioned on a rigid holder by means of a leaf spring pressing against the knife. A guiding means in this sharpener on one side of the disk edge limits the movement of the knife toward the disk. Even with these precautions, by deliberately avoiding planar contact with the disk face perpendicular to its axis of rotation there is only a point or limited line of contact between the blade and abrasive during sharpening and there is a strong tendency to gouge the knife edge. The abrasive passes the knife edge in essentially one fixed direction which leaves burrs and unacceptable large serrations on the blade edge.
A common version of this approach is described in U.S. Pat. No. 2,775,075 where the edge of the abrasive disk is beveled to enlarge the line of contact along that bevel of the knife edge with the abrasive. The tendency of such sharpeners to gouge knife blades is well known and at best the resulting knife edge is poorly defined and serrated. In all such sharpeners and abrasive passes the knife edge in essentially one fixed direction which creates the serrations and a sizeable burr on the knife edge.
A complex sharpener covered by U.S. Pat. No. 2,519,351 contains two pair, a total of four (4) abrasive blocks, one pair of which is biased to move toward the other, that sharpens by a reciprocating rectilinear motion simultaneously both cutting edge facets of a knife. The knife is held by three sets of jaws in a positioning means designed to be free floating in lateral position between the abrasive pairs and to moderate insertion of the blade into the positioning means by engaging the sides of the knife in one or more of three (3) grooved blocks. In addition to its complexity this sharpener has the disadvantages inherent in all rectilinear motion sharpeners which leaves a serrated knife edge which cuts by tearing and has poor wear characteristics. The free floating design of the positioning means and the inherent tendency of the two cutting edge facets of the blade to jam in the grooved block makes this inapplicable in virtually any other sharpener. Because both sides of the knife of sides of its cutting edge facets are used to moderate the degree of knife insertion into the sharpener, and because of the free floating lateral motion, this prior art positioning means is inapplicable where a precise positioning of the knife edge is necessary. The degree of insertion of the knife edge and hence its position depends on the width of the knife, on the width and angle of its cutting edge facet and on the degree of manual pressure applied during insertion and movement of the knife.
U.S. Pat. No. 2,751,721 describes a sharpener with a drum shaped abrasive element where the knife cutting edge facet is sharpened against annular portion of the drum surface that rotates in a plane perpendicular to the axis of rotation of the drum. The abrading force on the cutting edge is determined solely by the degree of hand pressure applied to the knife by the operator which leads to significant inconsistencies in abrading rate, poor edge formation, and gouging of the edge -- problems common to much of the prior art. Position and stability of the knife within the holder and angular control of the cutting edge facet against the abrasive surface is poor because of their dependency on the amount of pressure applied by the operator and by the profile of the several bevel faces common to the existent variety of commonly available knives.
U.S. Pat. No. 2,645,063 describes a sharpener with a drum surface and a guide mechanism which provides stops that position the knife by bearing directly on the cutting edge itself. Such stops are impractical because of the constant dulling effect on the edge created by rubbing it directly across and normal to one surface of the guide. This patent and U.S. Pat. No. 2,751,721 describe sharpeners that incorporate a magnet. The magnetic field does not support or guide the knife.